External scanning is just part of the pilot's total eyeball job. To achieve maximum
efficiency in flight, one has to establish a good internal (panel) scan as well and
learn to give each instrument its proper share of time. The amount of time one spends
eyeballing outside the cockpit in relation to what is spent inside depends,
to some extent, on the workload inside the cockpit and the density of traffic
outside. Generally, the external scan will take about three to four times as long
as a look around the instrument panel.

A major company conducted an experimental scan training course, using military
pilots ranging in experience from 350 to over 4,000 hours. They discovered that
the average time needed to maintain a flight situation status quo was
three seconds for panel scan and 17 seconds for outside.
(Since military pilots are most likely flying a more consistent schedule than most
general aviation pilots, we should allow six or seven seconds on the panel.)